The present invention relates to the writing on a support of elements of information capable of being read optically. It relates more particularly to a thermo-optic writing process and a thermo-sensitive support which allows the elements of information to be read immediately after recording thereof without it being necessary to provide a treatment such as chemical or heat development.
The use of a focused laser beam has been proposed as a writing means in combination with information support structures comprising generally thin-layer materials of a thickness between 30 and 100 nm. In known constructions, the surface energy density used to obtain for example the localized thermal ablation of a layer is relatively high. For the layer, semi-metals with a low melting point such as bismuth or tellurium have been proposed. However, experience shows that these semi-metals only give impressions with reproducible edges if the size of these impressions is sufficient, which greatly limits the storage capacity of the material. As for the surface energy density required for forming a permanent impression, it is normally greater than 80 mJ.cm.sup.-2 so that low-power lasers cannot be envisaged when the flow of information is greater than a few M bits s.sup.-1.
Besides semi-metals, there exist materials operating by thermal ablation such as vitreous chalcide alloys which have a lower thermal diffusivity and whose layers are more isotropic. These materials offer a greater sensitivity and allow impressions to be obtained by ablation having a more reproducible profile, even on the scale of 0.5 .mu.m. However, here again, because of the thermal ablation process itself, the cup-shaped impressions are surrounded by a rim which is the cause of considerable noise which is superimposed on the reading signal.
Instead of using as information support a substrate having a single thermosensitive layer it has been contemplated covering a thermodegradable organic layer with a thin metal layer. In this case, the radiated writing energy is converted into heat energy by the metal layer and the heat thus created serves to produce a change of state localized in the thermodegradable layer. The products of decomposition create a gassy microbubble capable of piercing the metal layer and etching it. Here again, the impressions obtained have an uneven edge which does not lend itself to the storage of good-quality video signals.